Heretofore, certain ink jet systems have relied upon inductive charging of electrically conductive ink fluids in order to project charged ink droplets upon a printing target. Such systems are well known in the art, and all are fluid dependent, i.e. they require an ink fluid having a minimum electrical conductivity in order to adequately charge and project the ink fluid. These systems generally comprise a two electrode, diode type-structured inductive charging system. A typical prior art system of the aforementioned type is illustrated in U.S. Pat. No. 4,220,958 issued: Sept. 2, 1980.
In these diode type devices, the conductive ink flows through an orifice which is usually grounded. After exiting the orifice and while still a continuous columnar jet, the stream passes coaxially without physical contact through a second, usually cylindrical, electrode. This electrode is at a different potential from the orifice and the conductive ink liquid. As a result, an induced current flows through the ink to the protruding liquid column, and excess charge (of sign opposite to the cylindrical electrode) is in the fluid.
The exiting column breaks into droplets by electrohydro-dynamic, fluid-dynamic, mechanical or other means, thereby isolating the charge on the droplets. In order for the inductive charging process to work, it is essential that the fluid (ink) have sufficient electrical conductivity to permit adequate current to flow in the exiting jet and appropriate levels of charge to accumulate. Therefore, these systems are critically dependent upon the innate electrical conductivity of the ink for their operation.
The present invention features an entirely new approach to ink jet printing. The subject invention has its roots in research involving the atomization of fluids, and the developed theory supporting the electrostatic spraying of these fluids. Dr. Arnold J. Kelly, the present inventor, has pioneered this research at the Exxon Research and Engineering Laboratories in Linden, N.J., and is the proud holder of U.S. Pat. No. 4,255,777 issued: Mar. 10, 1881, entitled: "Electrostatic Atomizing Device". Dr. Kelly is also the author of the following articles: "Electrostatic Metallic Spray Theory", Journal of Applied Physics, Vol. 47, No. 12, December 1976; and "Electrostatic Spray Theory", Journal of Applied Physics, Vol. 49, No. 5, May 1978.
Inasmuch as this patent, and these articles may prove helpful in understanding the present invention, the teachings advanced therein are meant to be incorporated herein by way of reference.
By contrast, the charge injection process proposed by this invention can charge non-conductive and poorly conductive liquids as well as conductive liquids. In the inventive system, two electrodes are in contact with the liquid and are submerged by the liquid. One electrode is an emitter and serves to field emit charge into the liquid in response to a voltage difference imposed between it and the other (blunt) submerged electrode. Depending upon the electrical mobility of the fluid, the injected charge will be more or less trapped in the fluid and swept to the outside by the bulk motion of the fluid (ink). Once free of the dual electrode charging station, the exiting stream can be made to undergo breakup in a similar manner as that described for the aforementioned inductive system. The charge is thereby trapped on individual droplets. The paper or target upon which the droplets impinge functions as the third electrode, returning the charge and completing the circuit. The system as described, represents a triode-structured system, which to the best of our knowledge and belief is entirely new within the art.
Additional mechanical or vibrational pulsing of the ink fluid may be used to project ink droplets from an orifice in a traditional droplet formation scheme, with the charge injection functioning as a means to control droplet formation and direction.
The charge injection process is of particular interest because it is: (a) essentially independent of fluid conductivity; and (b) compact and capable of modest voltage operation.
It should be noted that the field emitter, dual submerged electrode geometry described, is but one of a very broad class of possible devices that can be used to charge inject liquids. For instance, a conventional thermionic vacuum electron gun, firing through an appropriate window can be used to charge the flowing ink stream prior to exiting the head. Therefore, the invention is not to be limited by any specific exposition, description of which is exemplary and meant only to convey an understanding of the invention.